[DonRotolo]

Indeed, the exact specs of pump and panel will allow for intelligent answers.

Often, solar panels are rated in a funny kind of way: The peak voltage is measured open-circuit, and the peak current is measured short-circult. That means that a panel rated this way for 20 V DC and 2 A will produce neither under normal conditions.

A 20 V panel is usually used for a nominal 12 volt system (13.8 volts, as in automotive). It is probable that you will be unable to get 1.8 Amps out of it in full sunlight while maintaining 13.8 volts.

My recommendation would be to find a different motor that runs on 12 V DC at about 1.2 Amps. Then, connect a battery* in parallel to account for clouds and other short-term shadows. That will give reasonable results.

*NiCd batteries, like a bank of "D" cells, will provide the best results, long-term.

If you must use the 12 V AC motor, the idea is to use an electronic circult like a 555 timer to switch a transistor on and off at the desired frequency (likely 60 Hz), which will turn the DC into AC. Some low-pass filters you make will help shape the ugly triangle or square wave into a reasonable sine wave and you'll be good to go. None of this is difficult or expensive for the voltages and currents involved here.